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Can MD Simulations of Solvated Bis(L- isoleucinato)copper(II) in Aqueous Solution Explain a Disorder in the X-Ray Crystal Structure

The X-ray crystal and molecular structure of cis-aquabis(Lisoleucinato) copper(II), Cu(L- Ile)2∙H2O, obtained by recrystallisation from the mixture of acetic acid and water, and measured at 120 K and 295 K is intrinsically disordered in the crystalline state. This intrinsic disorder makes crystal structure refinement rather difficult, giving only limited information about the molecular structure. While the conformation of one L- isoleucinato ligand is always the same, we identified at least three different conformations of the second L-isoleucinato ligand in the crystal structure. Encouraged by the results of the MD simulations of solvated trans and cis bis(glycinato)copper(II) [1] and bis(L-valinato)copper(II) systems [2] in aqueous solutions that supported the experimental evidences, we performed the 20 ns of MD simulations of the Cu(L-Ile)2∙3435 H2O system in aqueous solution at room temperature. The simulations were done for trans and cis isomers of Cu(L-Ile)2 separately, and started from all possible trans and cis conformers having the same two chelate-ring conformations. Although in vacuo the trans-conformers are predicted to be more stable than the cis-ones, more favourable intermolecular interactions of the cis- than the trans-conformers with the water molecules predict the occurrence of both isomers in aqueous solution. The cis conformers that have the most favourable intermolecular interactions with the water molecules are the ones that were identified to be present in the crystal structure. Based also on the MD simulation results obtained for the solvated trans- and cis-bis(L-valinato)copper(II) systems [2], we suppose that the most favourable intermolecular interactions between a copper(II) complex and water molecules can be used to predict molecular conformations obtained by crystallisation from aqueous solutions. Crystal structure predictions for the trans and cis conformers of Cu(L-Ile)2 were calculated in order to examine the MD results.

copper; isoleucine; force field; crystal simulation; disorder; MD

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